1. Field of the Invention
The present invention relates to a lithium (Li) secondary cell, and more particularly, to a lithium (Li) composite oxide, a preparation method thereof, and a Li ion secondary cell adopting the Li composite oxide as an active material of a positive electrode.
2. Description of the Related Art
As small, light, wireless electronic devices, such as camcoders, cellular phones and notebook computers, have been introduced into the market, a small, light secondary cell having high energy density has been required as a power source for these devices. In this aspect, a lithium secondary cell has been spotlighted.
Lithium can be used as a negative electrode material of a battery because its electronegativity is large and it has the largest electric capacity per unit weight due to its low molecular weight. However, lithium in the metal state tends to undergo dendrite crystal growth while being passivated by the reaction with an organic solvent, causing a short within the cell. Thus, there is a problem of stability of a cell. Thus, as a negative electrode material capable of replacing lithium metal, carbonaceous material, which has the most similar electric potential to lithium, and allows a reversible intercalation/deintercalation of the lithium ions due to its layered structure, has been developed.
Electrode reaction in the lithium secondary cell constituted of a carbonaceous negative electrode, a metal oxide positive electrode and a liquid electrolyte is as follows.
During the charging period, lithium ions of the positive electrode are deintercalated, and the lithium ions of the electrolyte solution are intercalated into the layered structure of the carbonaceous material, so that the concentration of lithium ions within the electrolyte solution is constantly maintained. During the discharging period, intercalation/deintercalation of the lithium ions is performed in the reverse direction to the charging period. The cell is called a xe2x80x9crocking chairxe2x80x9d battery because the lithium ions reciprocate between two electrodes during the charging/discharging periods. Also, lithium exists as ions, without participation of the lithium metal itself, thus the cell is called a xe2x80x9clithium ion cellxe2x80x9d
In the above lithium ion secondary cell, the Li metal oxide is used as the positive electrode material. Particularly, LiCoO2, LuMn2O4, LiNiO2, etc. have been used as the positive electrode material. Even though a lithium oxide containing cobalt has been commercialized, cobalt is deleterious and expensive. On the contrary, the lithium oxide containing nickel provides a high capacity and less deleteriousness at low costs. However, it is difficult to synthesize lithium oxide in a power form, and the lift span is not so good. To solve the above problems, a lithium composite oxide expressed by LiMMxe2x80x2Ox (here, M and Mxe2x80x2 are transition metals, independently selected from cobalt (Co), manganese (Mn), nickel (Ni), vanadium (V), iron (Fe) and tungsten (W). Particularly, Ni of LiNiO2 is partially substituted with another metal, so that the synthesis becomes easy and the life span is improved. Generally, such improvement is achieved by the crystalline structure or the particle shape of the lithium oxide. For example, in the case where the particle shape of the positive electrode active material is irregular in shape and small (approximately 5 xcexcm of average diameter), a cell having high capacity can be obtained due to the smooth intercalation/deintercalcation of the lithium ions. Also, when the particle shape of the positive electrode active material is close to a spherical shape, which is advantageous to increase tap density, thus the relative weight ratio of the positive electrode active material can be increased in the preparation of the positive electrode active material.
Japanese Patent laid-open Publication No. Heisei 7-37576 discloses a method of preparing lithium oxide containing nickel. According to the disclosure, plate type monocrystalline particles (primary particles) of nickel hydroxide, educed by the neutralization reaction between a Ni salt solution and an alkaline solution, are agglomerated to form spherical or elliptic secondary particles. Then, the secondary particles of the nickel hydroxide and lithium compound, and magnesium compound if required, are thermally treated under an oxygen atmosphere, thereby forming a lithium compound containing nickel. In the case of the lithium oxide containing nickel, obtained by the above method, the flat type primary structure of the lithium oxide, where a plurality of flakes are stacked to form a layered structure, agglomerate to form the spherical or elliptical secondary structure having approximately 2xcx9c20 xcexcm average diameter. That is, the secondary structure of the particles is obtained through the assembling process to agglomerate the primary particles having a layered structure. However, since the secondary particles are formed by agglomerating the primary particles through the assembly process, it is difficult to make the active material fine, and special attention is required to optimize the assembling conditions.
Japanese Patent laid-open Publication No. Heisei 8-339806 discloses a lithium composite oxide containing nickel represented by LiNi(1-x)MxO2 (here, M is Co or Al, and x=0.05xcx9c0.3). According to this disclosure, an alkaline solution is added to a mixed aqueous solution containing cobalt salt and nickel salt to coprecipitate cobalt hydroxide and nickel hydroxide, resulting in a composite hydroxide. By agglomerating the monocrystalline particles of the composite hydroxide into a spherical or elliptic shape, a secondary structure of the particles is formed, and then lithium compound is added to the composite hydroxide and then thermally treated to form a lithium composite oxide containing cobalt in which the layered structure of the particle is exposed toward the outside of the spherical or elliptic secondary structure of the particles. According to this method, spherical particles having the secondary structure are formed at the point in time when the Ni and Co composite hydroxide is formed. Then, a lithium compound is added and then a thermal treatment is performed while the shape of the particle is maintained without changes. However, while the capacity is improved by the thermal treatment at approximately 750xc2x0 C., it is difficult to maintain the shape of the particles as the treatment temperature increases. Also, because the layered structure of the particle is exposed toward the outside of the secondary structure of the particle, the tap density is not high.
To solve the above problems, it is an objective of the present invention to provide a simple method of preparing a lithium composite oxide without an assembling process.
It is another objective of the present invention to provide a lithium composite oxide prepared by the method.
It is still another objective of the present invention to provide a lithium secondary cell adopting the lithium composite oxide as an active material of a positive electrode.
Accordingly, to achieve the above first objective, there is provided a method of preparing a lithium composite oxide (LiaNi(1-x)CoxO2, where a=0.97xcx9c1.05 and x=0.1xcx9c0.3), comprising the steps of: (a) coprecipitating a Nixe2x80x94Co composite hydroxide by adding an aqueous ammonia solution as a complexing agent, and an alkaline solution as a pH-adjusting agent, to an aqueous mixed solution containing a cobalt salt and a nickel salt; (b) adding lithium hydroxide to the composite hydroxide and thermally treating the mixture at 280xcx9c420xc2x0 C.; and (c) thermally treating the resultant of the step (b) at 650xcx9c750xc2x0 C.
Preferably, in the step (a), at least one metal salt selected from the group consisting of aluminum (Al), calcium (Ca), magnesium (Mg) and boron (B) is mixed at a mole ratio of 0.1:1 or less based on the total metal salt, before the complexing agent and the pH-adjusting agent are added to the acqueous mixed solution containing the Ni salt and Co salt.
Preferably, the coprecipitation of the step (a) is performed for 30xcx9c180 minutes such that a secondary particle having an irregular structure formed of acicular primary particles is formed.
Preferably, the coprecipitation reaction of step (a) is performed for 6xcx9c10 hours.
Preferably, both thermal treatment steps (b) and (c) are performed under a dry atmosphere.
To achieve the second objective, there is provided a lithium composite oxide (LiaNi(1-x-y)CoxMyO2, where M is at least one metal atom selected from the group consisting of Al, Ca, Mg and B, a=0.97xcx9c1.05, x=0.1xcx9c0.3, and y=0xcx9c0.05) in which primary particles having a square structure are agglomerated to form a spherical secondary particle, and having a tap density of 2.4xcx9c3.2 g/cm3.
Preferably, the primary particle has a triangle, square or pentagon shape, particularly, square shape, and an average particle diameter of 0.2xcx9c0.5 xcexcm, and the secondary particle has an average particle diameter of 5xcx9c25 xcexcm.
Preferably, the primary particle has an average particle diameter of 0.2xcx9c0.5 xcexcm, and the secondary particle has an average particle diameter of 2xcx9c6 xcexcm.
To achieve the third objective, there is provided a lithium ion secondary cell comprising a positive electrode containing a lithium oxide as an active material, a negative electrode containing a carbonaceous material and a non-aqueous electrolyte, wherein in the lithium oxide of the positive electrode, primary particles having a square structure are agglomerated to form a spherical secondary structure. Here, the lithium oxide of the positive electrode is a lithium composite oxide (LiaNi(1-x-y)CoxMyO2 where M is at least one metal atom selected from the group consisting of Al, Ca, Mg and B, a=0.97xcx9c1.05, x=0.1xcx9c0.3, and y=0xcx9c0.05), having a tap density of 2.4xcx9c3.2 g/cm3.
Also, there is provided a lithium composite oxide (LiaNi(1-x-y)CoxMyO2 where M is at least one metal atom selected from the group consisting of Al, Ca, Mg and B, a=0.97xcx9c1.05, x=0.1xcx9c0.3, and y=0xcx9c0.05) in which primary particles having a square structure are agglomerated to form an irregular secondary particle.